I've installed Outbrain on several of my WP web sites, and I'm always impressed by how easy it is to use and implement. In addition, Outbrain's a great way to get extra visits to or clicks on my content, whether it's from my own sites or other sites in the network as a recommendation. Review collected by and hosted on G2.com.
The traffic I get from Outbrain visitors tends to not be very sticky. (As in, people will come to the site through the recommendation widget, and then tend to only view that one article - and for not much time to boot.) That's not indicative of Outbrain per se, but the platform category. Review collected by and hosted on G2.com.